(Opto)electronic arrangements are being used with ever-increasing frequency in commercial products. They comprise organic or inorganic electronic structures, examples being organic, organometallic or polymeric semiconductors or else combinations of these. Depending on the desired application, they are rigid or flexible in form, there being an increasing demand for flexible arrangements.
Examples of (opto)electronic applications that are already commercial or are of interest in terms of their market potential include electrophoretic or electrochromic constructions or displays, organic or polymeric light-emitting diodes (OLEDs or PLEDs) in readout and display devices or as illumination, electroluminescent lamps, light-emitting electrochemical cells (LEECs), organic solar cells, preferably dye or polymer solar cells, inorganic solar cells, preferably thin-film solar cells, more particularly those based on silicon, germanium, copper, indium and/or selenium, organic field-effect transistors, organic switching elements, organic optical amplifiers, organic laser diodes, organic or inorganic sensors or else organic- or inorganic-based RFID transponders.
A perceived technical challenge for realization of sufficient lifetime and function of (opto)electronic arrangements in the area of organic and/or inorganic (opto)electronics, especially in the area of organic (opto)electronics, is the protection of the components they contain against permeants. Permeants may be atoms, ions or a large number of low molecular mass organic or inorganic compounds, more particularly water vapor and oxygen.
A large number of (opto)electronic arrangements in the area of organic and/or inorganic (opto)electronics, especially where organic raw materials are used, are sensitive not only to water vapor but also to oxygen, and for many arrangements the penetration of water vapor is classed as a relatively severe problem. During the lifetime of the electronic arrangement, therefore, it requires protection by means of encapsulation, since otherwise the performance drops off over the period of application. For example, oxidation of the components, in the case of light-emitting arrangements such as electroluminescent lamps (EL lamps) or organic light-emitting diodes (OLEDs) for instance, may drastically reduce the luminosity, the contrast in the case of electrophoretic displays (EP displays), or the efficiency in the case of solar cells, within a very short time.
In organic and/or inorganic (opto)electronics, particularly in the case of organic (opto)electronics, there is a particular need for flexible bonding solutions which constitute a permeation barrier to permeants, such as oxygen and/or water vapor. The flexible bonding solutions are therefore intended not only to achieve effective adhesion between two substrates, but also, in addition, to fulfill properties such as significantly improved permeation barrier properties, high temperature stability, high shear strength and peel strength, chemical stability, aging resistance, high transparency, ease of processing, and also high flexibility and pliability. In order to be able to ensure a very broad spectrum of application for barrier adhesives, there is also an increasing requirement for improved barrier properties, preferably coupled with a high temperature stability. This combination is needed when, for example, (opto)electronic components are encapsulated which become warm or hot during their operation, whether as a result of the development of heat or because of the surroundings in which they are used. The use of barrier adhesive for producing solar panels or else electronic components which are employed in offshore parks poses particular challenges to the temperature stability and barrier properties of the adhesive.
In order to obtain the most effective sealing, specific barrier adhesives are used. A good adhesive for the sealing of (opto)electronic components has a low permeability for oxygen and particularly for water vapor, has sufficient adhesion to the substrate, and is able to flow well onto the substrate. Owing to incomplete wetting of the surface of the substrate and to pores that remain, low capacity for flow on the substrate may reduce the barrier effect at the interface, since it permits lateral ingress of oxygen and water vapor independently of the properties of the adhesive. Only if the contact between adhesive and substrate is continuous are the properties of the adhesive the determining factor for the barrier effect of the adhesive.
For the purpose of characterizing the barrier effect it is usual to state the oxygen transmission rate OTR and the water vapor transmission rate WVTR. Each of these rates indicates the flow of oxygen or water vapor, respectively, through a film per unit area and unit time, under specific conditions of temperature and partial pressure and also, optionally, further measurement conditions such as relative atmospheric humidity. The lower the values the more suitable the respective material for encapsulation. The statement of the permeation is not based solely on the values of WVTR or OTR, but instead also always includes an indication of the average path length of the permeation, such as the thickness of the material, for example, or a standardization to a particular path length.
The permeability P is a measure of the perviousness of a body for gases and/or liquids. A low P value denotes a good barrier effect. The permeability P is a specific value for a defined material and a defined permeant under steady-state conditions and with defined permeation path length, partial pressure and temperature. The permeability P is the product of diffusion term D and solubility term S:P=D*S 
The solubility term S describes in the present case the affinity of the barrier adhesive for the permeants. In the case of water vapor, for example, a low value for S is achieved by hydrophobic materials. The diffusion term D is a measure of the mobility of the permeant in the barrier material, and is directly dependent on properties, such as the molecular mobility or the free volume. Often, in the case of highly crosslinked or highly crystalline materials, relatively low values are obtained for D. Highly crystalline materials, however, are generally less transparent, and greater crosslinking results in a lower flexibility. The permeability P typically rises with an increase in the molecular mobility, as for instance when the temperature is raised or the glass transition point is exceeded.
A low solubility term S is usually insufficient for achieving good barrier properties. One classic example of this, in particular, are siloxane elastomers. The materials are extraordinarily hydrophobic (low solubility term), but as a result of their freely rotatable Si—O bond (large diffusion term) have a comparatively low barrier effect for water vapor and oxygen. For a good barrier effect, then, a good balance between solubility term S and diffusion term D is necessary.
Approaches at increasing the barrier effect of an adhesive must take account of the two parameters D and S, with a view in particular to their influence on the permeability of water vapor and oxygen. In addition to these chemical properties, thought must also be given to consequences of physical effects on the permeability, particularly the average permeation path length and interface properties (flow-on behavior of the adhesive, adhesion). The ideal barrier adhesive has low D values and S values in conjunction with very good adhesion to the substrate.
For this purpose use has hitherto been made in particular of liquid adhesives and adhesives based on epoxides (WO 98/21287 A1; U.S. Pat. No. 4,051,195 A; U.S. Pat. No. 4,552,604 A). As a result of a high degree of crosslinking, these adhesives have a low diffusion term D. Their principal field of use is in the edge bonding of rigid arrangements, but also moderately flexible arrangements. Curing takes place thermally or by means of UV radiation. Full-area bonding is hard to achieve, owing to the contraction that occurs as a result of curing, since in the course of curing there are stresses between adhesive and substrate that may in turn lead to delamination.
Using these liquid adhesives harbors a series of disadvantages. For instance, low molecular mass constituents (VOCs—volatile organic compounds) may damage the sensitive electronic structures in the arrangement and may hinder production operations. The adhesive must be applied, laboriously, to each individual constituent of the arrangement. The acquisition of expensive dispensers and fixing devices is necessary in order to ensure precise positioning. Moreover, the nature of application prevents a rapid continuous operation, and the laminating step that is subsequently needed may also make it more difficult, owing to the low viscosity, to achieve a defined layer thickness and bond width within narrow limits.
Furthermore, the residual flexibility of such highly crosslinked adhesives after curing is low. Use of 2-component systems is limited by the potlife, in other words the processing life until gelling has taken place.
Particularly if the (opto)electronic arrangements are to be flexible, it is important that the adhesive used is not too rigid and brittle. Accordingly, pressure-sensitive adhesives (PSAs) and heat-activatedly bondable adhesive sheets are particularly suitable for such bonding. In order to flow well onto the substrate but at the same time to attain a high bonding strength, the adhesives ought initially to be very soft, but then to be able to be crosslinked. As crosslinking mechanisms it is possible, depending on the chemical basis of the adhesive, to implement thermal cures and/or radiation cures.
DE 10 2008 060 113 A1 describes a method for encapsulating an electronic arrangement with respect to permeants, using a PSA based on butylene block copolymers, more particularly isobutylene block copolymers, and describes the use of such an adhesive in an encapsulation method. In combination with the elastomers, defined resins, characterized by DACP and MMAP values, are preferred. The adhesive, moreover, is preferably transparent and may exhibit UV-blocking properties. As barrier properties, the adhesive preferably has a WVTR of <40 g/m2*d and an OTR of <5000 g/m2*d bar. In the method, the PSA may be heated during and/or after application. The PSA may be crosslinked—by radiation, for example. The PSA is not temperature-stable.
JP 4,475,084 B1 teaches transparent sealants for organic electroluminescent elements, that may be based on block copolymer. Examples listed are SIS and SBS and also the hydrogenated versions. Not specified, however, are constituents which permit crosslinking after application. Nor are the barrier properties of the sealants addressed. The sealing layer apparently does not take on any specific barrier function.
Additionally described are barrier adhesives based on styrene block copolymers, very substantially hydrogenated and correspondingly hydrogenated resins, DE 10 2008 047 964 A1.
Not only the PSAs based on polyisobutylene but also those based on hydrogenated styrene block copolymers exhibit a significant disadvantage. In the case of bonding between two films provided with a barrier layer, as for example two PET films with an SiOx coating, as may be used, for example, for organic solar cells, severe blistering occurs in the course of storage under humid and hot conditions. Even prior drying of the films and/or adhesive is unable to prevent this blistering.
A particular problem is that in general any kind of functionalization (whose purpose is to provide reactivity) causes an increase in basic polarity and hence an unwanted rise in the water vapor permeability of the adhesive.